1.while

map<int, int>::iterator iter;
    iter = _map.begin();
    while(iter != _map.end())
    {
        cout << iter->first << ' ' << iter->second << endl;
        iter++;
    }

2.本质上和第一种一样(for)

map<int, int>::iterator iter;
for(iter = _map.begin(); iter != _map.end(); iter++) 
{
    cout << iter->first << ' ' << iter->second << endl;
}

3. auto

for(auto v:_map)
    cout << v.first << ' ' << v.second << endl;